Why is the Z-seam always on the outside?

[T9_AIR 0]

Whenever I print a round object, such as a ball, the z-seam always ends up on the outside wall, and never on the inside. From what I have seen, the seam must end up on a vertex, so if you have a box, why can't the seam be on the inner corner? 

[GregValiant 1,255]

The seam is a function of FDM printing.  There will be a point on the outer wall where the nozzle starts to extrude it.  The Nozzle travels around extruding the outer wall, and then ends right where it started.  Those starts and ends are what makes the seam visible.

 

There is an Inner-Wall seam, a Support seam, an Infill Seam, a Prime Tower seam, and a Support Interface seam.  Even an "Ooze Shield" would have a seam.  Only the Outer Wall seam is displayed because it's the only one visible on a finished print.  The preview would get kind of cluttered if they were all shown.